Acute onset demyelinating encephalomyelitis with morphologic features similar to multiple sclerosis was first observed in group-housed Japanese macaques in 1986. The incidence of the disease peaked the following year, affecting 3% of the population. Subsequently, approximately 1% of the population developed demyelinating encephalomyelitis annually. The age of the affected animals ranged from 97 days to 20 years, with equal sex distribution. Using long-term explant and co-culture techniques, a highly cell-associated spumaretrovirus and a novel gamma-2 herpesvirus were recovered from lesioned brain and spinal cord. No viruses were isolated from CNS explant cultures from normal controls. To prospectively evaluate the relationship between spumaretrovirus and/or herpesvirus infection and encephalomyelitis, virus isolation from peripheral blood leukocytes was conducted on 203 animals in the troop in 1994; spumaretrovirus was isolated from 32 (15.8%). Of the 32 infected animals, 24 (75%) were less than 7 years of age. Virus isolations were repeated on 228 animals in 1995 and 209 animals in 1996. Spumaretrovirus was isolated from 43 (18.9%) in 1995 and 19 (9.4%) in 1996. Gamma herpesvirus was recovered from 10 (4.4%) in 1995 and 15 (7.5%) in 1996. Two (1.0%) animals in 1996 were infected with both agents. Four animals developed demyelinating encephalomyelitis in 1995-1996. All were herpesvirus-infected animals and herpesvirus was isolated from lesioned brain and spinal cord in three of the animals; only spumaretrovirus was isolated from the fourth animal. The prospective epidemiology study and experimental transmission studies will be used to further explore the etiology of this disease.